A capstan seems like the most reliable way to go. It’s easier to control a single, steady motor and keep track of its rotation than to use a direct-drive motor at the take-up reel, which is how I was doing it in version 1.
In other words, a motor near the gate that turns at a constant speed to advance the film. And then a separate motor at the take-up reel (see Take-Up Motors thread).
Keeping track of rotation would allow me to know when to take a photograph. This can be achieve with either a stepper motor (see this project) or by combining a regular motor with a rotary encoder. The stepper would seem to be the cheaper route and have less points of failure than a DC motor + rotary encoder.
My concern with a stepper is that over time there could be what I’m calling “frame float.” Typically when you use a stepper motor, you specify the degrees of rotation you would like it to turn each time it is activated. If 1 frame = 15 degrees (making that up for now), then every time I’ve turned 15 degrees, I have a new frame. Easy. However, if it’s mathematically 14.87 degrees for a truly perfect single frame advancement, then that 0.13 degree difference will add up with every frame that goes by.
Eventually, I’ll see the frame start to float to the left or right. this can affect the frame extraction software, which works best when it can assume the sprocket holes will always appear in the same place.
For this reason, I wonder if I will also need some feedback in the film path that can assure me I’m always capturing at the EXACT right moment.
Many have suggested a laser, or photo-interrupter. But this doesn’t work so well with clear celluloid. Any ideas on how to get around that could bring the laser option back into play.
Another thought was a really geared down (or up) rotary encoder. A small roller that is directly attached to a large wheel underneath the main deck plate. That large plate will exaggerate the smallest of turns into a long radial arc that can be monitored by optical encoders, switches, lasers, etc. Perhaps that will increase the accuracy to a rate that is acceptable over the time it takes to capture one full reel of film. This is what I’m thinking of pursuing at the moment.
Summary - a capstan on the right (exit) side of the gate would pull film through at a constant speed. Improvements might include trying a dual capstan (one before and one after the gate), and pinch rollers to maintain reliable constant speed and tension.
Just for fun, here's a picture of a reel-to-reel tape head that uses dual pinch rollers at the "gate."